Hematite is an iron oxide mineral widely found in rocks and soils across the globe. Its name comes from the Greek word haima, meaning blood — this refers to the red streak it leaves when rubbed against a harder surface. In fact, most red pigments in the world are derived from hematite. The world would literally be less colorful without this remarkable mineral.
Hematite also has lots of practical uses as the principal source of iron ore, without which there would be no steel industry. Let’s delve deeper.
How hematite forms
Direct Precipitation in Water
One of the primary methods by which hematite forms is through direct precipitation from iron-rich water in lakes, springs, or marine environments. This process occurs when iron dissolved in water oxidizes, forming solid particles of hematite that settle out of the water and accumulate on the bottom. Over time, these particles can form significant deposits, especially in areas where conditions favor the preservation of these sediments.
Sedimentary Processes
In sedimentary environments, hematite often forms as a result of the chemical weathering of iron-bearing minerals. As rocks containing iron minerals like magnetite or siderite are exposed to the atmosphere, they undergo oxidation, a process where iron reacts with oxygen to form hematite. This hematite can then be transported and deposited by water or wind, forming sedimentary layers that, over millions of years, become compacted and cemented into rock.
Metamorphism
Hematite also forms through the metamorphism of iron-rich sediments under high pressure and temperature conditions deep within the Earth. During metamorphism, the crystal structure and composition of the original sedimentary rock are transformed, leading to the formation of new minerals, including hematite. This process can result in the creation of banded iron formations (BIFs), which are layers of hematite and silica that are important sources of iron ore.
Volcanic Activity
Volcanic activity is another source of hematite. When volcanoes erupt, they can release gases rich in sulfur and other elements that react with iron in the surrounding rocks or magma. This reaction can produce hematite, which may be deposited around volcanic vents or within the volcanic rocks themselves. Hematite formed in this way is often associated with other minerals produced by volcanic processes.
Hydrothermal Activity
Hematite can also precipitate from hot, iron-rich fluids that circulate through cracks and fissures in the Earth’s crust, a process known as hydrothermal activity. These fluids, heated by magma or the natural heat of the Earth, can leach iron from rocks and deposit it as hematite when they cool or react with other chemicals. Hydrothermal hematite is often found in veins or as coatings on rock surfaces.
Soil Weathering
In addition to these primary formation processes, hematite is a common byproduct of the weathering of iron-rich minerals in soils. This type of hematite gives a red color to many tropical and ancient soils, where it forms as a fine dust that coats other soil particles. Weathering processes are particularly important in forming the red coloration seen in desert landscapes and on the surface of Mars.
Geologically, large hematite deposits often occur in banded iron formations, revealing a history of ancient, iron-rich oceans on Earth. The gray hematite variant is typically found in places with still, standing water or mineral hot springs, such as those in Yellowstone National Park, North America, where it can precipitate out of water and collect in layers.
Additionally, hematite’s role extends beyond Earth. It’s present on Mars, where its formation suggests the existence of past water environments. This interplanetary discovery underscores hematite’s significance in unraveling both Earth’s and Mars’s geological histories.
Hematite Physical and Chemical Properties
Chemical Composition
Hematite is an iron oxide with the chemical formula Fe2O3, making it a major source of iron. This composition includes two iron atoms and three oxygen atoms, leading to its deep red or rusty color in powdered form. The presence of iron contributes to hematite’s weight and density, making it heavier than many other common rocks or minerals.
Color and Streak
Hematite exhibits a color range from black to steel or silver-gray, brown to reddish-brown, or red when it is found in its natural, unpolished state. However, its most diagnostic feature is not its external color but its streak. When hematite is dragged across an unglazed porcelain plate, it leaves a distinctly reddish-brown streak, regardless of the color of the mineral itself. This streak test is a key way to distinguish hematite from similar-looking minerals.
Hardness and Brittleness
On the Mohs scale of mineral hardness, hematite ranks at about 5.5 to 6.5, making it moderately hard but less hard than quartz. Despite being harder than pure iron, hematite is also notably brittle. If subjected to stress, it tends to break rather than bend or deform, contrasting with the malleability of metallic iron.
Magnetic Properties
Hematite is known for its weakly magnetic properties, a feature that distinguishes it from the strongly magnetic mineral magnetite. However, certain forms of hematite may contain enough magnetite to exhibit weak magnetism. This characteristic is particularly interesting because it varies with the form of hematite; for example, magnetized hematite known as “magnetic hematite” is often used in jewelry.
Density and Specific Gravity
Hematite’s specific gravity ranges from 4.9 to 5.3, making it denser than many other common minerals. This high density is a result of its iron content, contributing to its use in applications requiring heavy materials, such as radiation shielding or ballast in ships.
Crystal Structure
Hematite crystallizes in the trigonal system, forming structures that can be tabular, rhombohedral, or scalenohedral. It shares the same crystal structure as corundum (Al2O3) and ilmenite (FeTiO3), which allows for the formation of solid solutions at high temperatures. This crystalline structure contributes to hematite’s metallic luster and ability to form various shapes, including the popular “iron rose” formations.
Electrical Conductivity
Hematite possesses a degree of electrical conductivity. This property is due to the presence of iron, which allows for the transfer of electrons under certain conditions. The electrical conductivity of hematite can be utilized in various technological applications, including electrochemical devices and sensors.
Thermal Stability and Melting Point
Hematite is thermally stable up to temperatures of about 950°C, above which it can transform or react with other materials.
Solubility
Hematite is generally insoluble in water at neutral pH levels but can dissolve in strong acids. This chemical resistance contributes to its longevity as a pigment and its durability in various environmental conditions.
How to Identify Hematite
This practical guide aims to equip you with the knowledge and techniques to confidently recognize hematite.
Step 1: Examine the Color
- Look for colors ranging from metallic gray to black in specular hematite.
- Earthy hematite may appear red to reddish-brown, reflecting its iron oxide composition.
Step 2: Conduct a Streak Test
- Use a streak plate (unglazed porcelain tile) to scratch the mineral across.
- Hematite will leave a reddish-brown streak, regardless of the specimen’s surface color.
Step 3: Assess the Luster
- Determine if the luster is metallic, shiny, dull, or earthy.
- Specular hematite exhibits a metallic and shiny luster, while other forms may have a dull or earthy appearance.
Step 4: Check for Hardness
- Use simple tools like a steel knife (hardness of about 5.5) or a glass plate (hardness of about 5.5).
- Hematite should be able to scratch these materials, indicating its hardness is in the 5.5 to 6.5 range.
Practical Tips:
- Always have a streak plate handy for a quick and reliable test.
- Familiarize yourself with hematite’s various forms by comparing your findings to known samples or images.
- Wear gloves when handling rough specimens to avoid staining your hands with the mineral’s powder.
Uses for Hematite
- Iron and Steel Production:
- Core material for iron extraction and steel manufacturing.
- Integral in construction materials, vehicles, appliances, and tools.
- Pigments and Dyes:
- Used for red hues in paints, dyes, and cosmetics since ancient times.
- Historic significance in art and cultural expressions.
- Jewelry and Ornamental Items:
- Polished for use in necklaces, bracelets, earrings, and decorative pieces.
- Valued for its metallic sheen and purported healing properties.
- Radiation Shielding:
- High density makes it suitable for blocking radiation in medical and nuclear applications.
- Heavy Media Separation:
- Exploits high specific gravity for density-based filtering processes in mining and recycling.
Facts about Hematite
- Hematite has been used by human societies for thousands of years, from early cave paintings to Egyptian tombs, indicating its enduring value across cultures.
- The presence of hematite can signal past water activity, especially on Mars, where its discovery by space missions suggested historical water flows.
- Hematite exhibits several forms, including the shiny, metallic specularite and the red, earthy ochre, each with distinct appearances and uses.
FAQ about Hematite
- Is hematite magnetic? While hematite is weakly magnetic, it doesn’t exhibit the strong magnetism characteristic of some iron minerals like magnetite.
- Can hematite be used in jewelry? Yes, hematite is popular in jewelry, especially for its polished, metallic forms that make striking beads and accents.
- How can you tell real hematite? Real hematite will leave a reddish-brown streak when scratched on a piece of unglazed porcelain, distinguishing it from similar minerals.
- Why is hematite important? Hematite is crucial for iron production, which in turn is fundamental for manufacturing steel, the backbone of infrastructure and construction worldwide.
